This invention relates to a combustion system and more particularly to a combustion system utilizing a supercritical water/fuel composition.
The recently enacted clean air legislation has targeted fossil fuel emissions. This legislation has prompted both the engine manufacturers and the fuel providers to look for solutions to be able to continue selling their products. Refiners must look at alternative formulations and/or blends to reduce emissions. Engine designers on the other hand, must rethink the entire combustion process and how it is conducted from beginning to end.
Engine manufacturers are going to higher tolerances on the piston--wall machining to reduce engine oil burning. Additionally, they are moving to higher and higher injection pressures. The higher pressures result in better spray penetration into the combustion zone as well as finer droplet sizes. The higher pressures permit smaller orifices at the injector tips while still maintaining the same mass flow rate.
With newly developed injectors operating at up to 30,000 psi, the droplet size is reduced but the droplet dimensions are still in the 1-10 micron range. A droplet size reduction by a factor of two would necessarily be accompanied by an increase in droplet number by a factor of eight from a mass balance perspective. This is important because many small droplets improve the microscopic homogeneity and reduce particulate matter production. Unfortunately, the droplet size reduction at these extreme pressures is less than a factor of two over the standard 3800 psi systems.
By way of example, a 2 micron diameter droplet occupies 3.times.10.sup.-12 cm.sup.3 and one mole of diesel fuel occupies 300 cm.sup.3. Therefore, this size droplet still contains 10.sup.-14 moles of fuel molecules, or 6.times.10.sup.+9 molecules. It is clear that even these smaller droplets still present a challenge to completely evaporate and combust. An additional mechanism beyond pressure increases must be found.
It is well known in the art that droplet size is primarily related to the surface tension of the fluid. Therefore, any process that reduces the surface tension can potentially reduce the droplet size. A chemical approach to droplet size reduction can be found in surfactant technology. Surface tension reducing additives have been applied, but their efficacy has been limited by high cost. Clearly, other chemical approaches are called for.
It is well known in the art that addition of heat to a hydrocarbon fuel reduces its surface tension. Thus, preheating of the fuel has some appeal from both an emissions and fuel economy perspective. In practice however, heating leads to premature reformulation of the fuels into higher as well as lower molecular weight compounds. The fuel's viscosity increases at a rate that outpaces the drop in surface tension and a sticky, tarry residue is produced. Therefore, simple preheating of hydrocarbon fuels has limited use.
There are however, additives that can alter this tendency toward molecular weight increases upon heating. Water for example, promotes the opposite process whereby hydrocarbon fuels are reduced in molecular weight by partial conversion to H.sub.2 and CO gases and reduction of chain length. Therefore, water and fuels mixed together and heated offer an economical solution to droplet size reduction. Additionally, the reformulated H.sub.2 provides cetane enhancement as it possesses a wider flammability limit. H.sub.2 gas in limited quantities has beneficial qualities, because the wider flammability limits can ignite at lower oxygen concentrations. These H.sub.2 molecules thereby serve as combustion initiators that are well distributed in the reaction zone.
Water addition to heated fuels offers benefits, but water and hydrocarbons do not mix readily. The polar nature of water and the nonpolar character of fuels favors phase separation into two unmixed pure liquids. Water does not exhibit an antibonding interaction with fuels. It simply has an overwhelmingly strong attraction for other water molecules that precludes bonding with hydrocarbon units. This phase separation property can be ameliorated by the addition of surfactants and cosurfactants, but as already stated they are expensive.